Choosing an Auth Mechanism

Authentication and authorization for Google APIs allow third-party
applications to get limited access to a user's Google account for certain
types of activities. This document explains the available auth mechanisms and describes what each one provides for your application.

Google+ Sign-In provides a simple way to let people use their Google credentials to sign-in to your site. It includes a set of tools that are easy to integrate across different devices.

OAuth 2.0 is an authorization protocol for all Google APIs. OAuth 2.0 relies on SSL for security instead of requiring your application to do cryptographic signing directly. This protocol allows your application to request access to data associated with a user's Google Account.

Login with OAuth 2.0 (OpenID Connect) authenticates users by having them log in with their Google accounts. This is a replacement for OpenID, and users of OpenID should plan to migrate to Login with OAuth 2.0.

Deprecated ServicesAuthSub, ClientLogin and OAuth 1.0 are available as alternatives to OAuth 2.0 for some Google APIs. As announced in 2012, these deprecated services will be shutdown on April 20th, 2015. If you already have applications that use them, you should migrate to OAuth 2.0.